Quantification limits in analytical reports

ICH Q2(R2) defines the quantification limit in relation to suitable precision and accuracy. Detection answers another question: whether there is a distinguishable signal under the analytical conditions. A signal can be observable without its conversion into a quantity meeting the required performance.

‘Detected,’ ‘below LOQ’ and a quantified figure should therefore not be treated as equivalent labels. The report must preserve the distinction between analytical presence and assignment of quantity. Official guidance.

Eurachem distinguishes limits obtained by direct introduction into the instrument from limits representing the entire procedure. Preparation, matrix and process variability form part of the scope when the intention is to describe the complete method.

A value obtained with a simple solution should not be transferred without explanation to a sample undergoing extraction and other stages. The reader should ask what path the materials followed when the limit was established and whether that path represents their sample. Eurachem guide.

Q2(R2) considers different approaches to determining limits, including signal-to-noise ratios where appropriate. The document calls for presentation of both the limit and the approach used. An isolated figure loses its connection with the procedure that produced it.

This distinguishes a preliminary estimate from a limit substantiated for an analytical purpose. Nor does every number calculated by software automatically represent the limit applicable to the final report.

In bioanalysis, ICH M10 connects the lower limit with a calibration range and defined performance criteria. It also addresses circumstances in which a run's acceptable range changes when an extreme calibrator is rejected.

The limit published as a method characteristic and the range actually supported in a run must therefore be read together. This observation comes from bioanalytical guidance; it does not turn its specific criteria into universal rules for every chemical certificate. Official guidance.

Imagine two reports saying ‘below LOQ’ but using different limits. The samples cannot be concluded to have the same concentration. The label communicates a quantification constraint, not a common value hidden behind it.

A useful reading preserves analyte, unit, matrix, limit and explanation of the result. If it is compared with an acceptance criterion, both must be expressed on a compatible basis. LOQ describes measurement capability; an acceptance criterion asks whether the result meets a separate requirement.